The extendible linkage normally used on such a bellows camera comprises a pair of scissor links or lazytongs on opposite sides of its optical axis, e.g. at the top and at the bottom. Conventional devices for varying the distance of the lens mount from the camera housing usually coact with only one of these scissor links, which does not permit very accurate focusing. Other normal types of adjusting mechanisms, carried on the lens mount, coact with both scissor links but are somewhat difficult to manipulate.
One of the mechanical problems involved is the decoupling of the adjusting mechanism from the scissor links for the purpose of facilitating a collapsing of the camera, i.e. a retraction of the lens mount into the housing. In principle, of course, a scissor linkage could be driven beyond the normal focusing range (i.e. past the "infinity" position) to reduce the spacing of the lens mount and housing to almost zero. In practice, however, such a retraction of the lens mount through the articulated linkage is cumbersome and time-consuming, as is the subsequent re-extension of the lens mount for another round of picture-taking.